Methods and apparatus for aligning a set of patterns on a silicon substrate

ABSTRACT

The disclosure relates to a method of aligning a set of patterns on a substrate, which includes depositing on the substrate&#39;s surface a set of silicon nanoparticles, which includes a set of ligand molecules including a set of carbon atoms. The method involves forming a first set of regions where the nanoparticles are deposited, while the remaining portions of the substrate surface define a second set of regions. The method also includes densifying the set of nanoparticles into a thin film to form a set of silicon-organic zones on the substrate&#39;s surface, wherein the first and the second set of regions have respectively first and second reflectivity values, such that the ratio of the second reflectivity value to the first reflectivity value is greater than about 1.1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.12/468,540, filed May 19, 2009, the entire contents of which areincorporated herein by reference.

FIELD OF DISCLOSURE

This disclosure relates in general to Group IV semiconductors and inparticular to methods and apparatus for aligning a set of patterns on asilicon substrate.

BACKGROUND

Semiconductors form the basis of modern electronics. Possessing physicalproperties that can be selectively modified and controlled betweenconduction and insulation, semiconductors are essential in most modernelectrical devices (e.g., computers, cellular phones, photovoltaiccells, etc.). Group IV semiconductors generally refer to those firstfour elements in the fourth column of the periodic table: carbon,silicon, germanium and tin.

The ability to deposit semiconductor materials using non-traditionalsemiconductor technologies such as printing may offer a way to simplifythe fabrication process and hence reduce the cost of many modernelectrical devices (e.g., computers, cellular phones, photovoltaiccells, etc.). Like pigment in paint, these semiconductor materials aregenerally formed as microscopic particles, such as nanoparticles, andtemporarily suspended in a colloidal dispersion that may be laterdeposited on a substrate.

Nanoparticles are generally particles with at least one dimension lessthan 100 nm. In comparison to a bulk material (>100 nm) which tends tohave constant physical properties regardless of its size (e.g., meltingtemperature, boiling temperature, density, conductivity, etc.),nanoparticles may have physical properties that are size dependent, suchas a lower sintering temperature.

The nanoparticles may be produced by a variety of techniques such asevaporation (S. Ijima, Jap. J Appl. Phys. 26, 357 (1987)), gas phasepyrolysis (K. A Littau, P. J. Szajowski, A. J. Muller, A. R. Kortan, L.E. Brus, J Phys. Chem. 97, 1224 (1993)), gas phase photolysis (J. M.Jasinski and F. K. LeGoues, Chem. Mater. 3, 989 (1991);),electrochemical etching (V. Petrova-Koch et al., Appl. Phys. Lett. 61,943 (1992)), plasma decomposition of silanes and polysilanes (H. Takagiet al, Appl. Phys. Lett. 56, 2379 (1990)), high pressure liquid phasereduction-oxidation reaction (J. R. Heath, Science 258, 1131 (1992)),etc.

One such advantage is that Group IV semiconductor nanoparticle materialsoffer the potential of high volume, low-cost processing for the readydeposition of a variety of Group IV nanoparticle inks on a range ofsubstrate materials. After printing, a suitable fabrication method of aGroup IV semiconductor device, such as a range of optoelectric devices,including photovoltaic devices must be selected that is compatible withthe overall goal of high volume processing.

For example, a solar cell converts solar energy directly to DC (directcurrent) electric energy. Generally configured as a photodiode, a solarcell permits light to penetrate into the vicinity of metal contacts suchthat a generated charge carrier (electrons or holes (a lack ofelectrons)) may be extracted as current. And like most other diodes,photodiodes are formed by combining p-type and n-type semiconductors toform a junction. After the addition of passivation and anti-reflectioncoatings, a layer acting as back surface field and metal contacts(fingers and busbar on the emitter, and pads on the back of theabsorber) may be added in order to extract generated charge. Emitterdopant concentration, in particular, must be optimized for both carriercollection and for contact with the metal electrodes.

In general, a low concentration of dopant atoms within an emitter regionwill result in low recombination (thus higher solar cell efficiencies(the percentage of sunlight that is converted to electricity)), but poorelectrical contact to metal electrodes. Conversely, a high concentrationof dopant atoms will result in high recombination (reducing solar cellefficiency), but low resistance ohmic contacts to metal electrodes.Often, in order to reduce manufacturing costs, a single dopant diffusionis generally used to form an emitter, with a doping concentrationselected as a compromise between reducing recombination and improvingohmic contact formation. Consequently, potential solar cell efficiencyis reduced.

One solution is the use of a dual-doped or selective emitter. Aselective emitter uses a first lightly doped region optimized for lowrecombination, and a second heavily doped region pattern (of the samedopant type) optimized for low resistance ohmic contact formation.Selective emitters are commonly formed with either multiple diffusionsteps in conjunction with diffusion blocking layers, or with the use ofmultiple dopant sources. Commonly, the principal variation between suchregions is a difference in dopant atomic concentration, and there isgenerally no visible contrast between the highly and lightly dopedregions. Consequently, the alignment of a metal contact pattern onto apreviously deposited highly doped region pattern is a large technicalchallenge.

For example, the general lack of a visible contrast makes it difficultto monitor the accuracy of metal contact pattern placement or to detectpotential axial and/or angular misalignment.

Likewise, in a back-contact solar cell, a set of counter-dopedinter-digitated highly doped patterns with superimposed metal contactsare configured on the back side of the solar cell. In addition, the backsurface may also be lightly doped with one of the dopants used in theinter-digitated highly doped patterns to form a BSF (back surfacefield). As with selective emitters, the visual boundaries between highlydoped and lightly doped regions are difficult to determine.Consequently, metal contact pattern alignment for this cell structure isalso problematic.

Common alignment methods are substrate edge alignment or alignment tofiducial marks. Fiducial marks (or fiducials) allow a pattern depositiondevice, usually a screen printer or inkjet printer, to deposit thedesired pattern relative to specific coordinates on the solar cell.These fiducial marks are placed in an independent step beforepatterning, thus requiring extra tools and processing steps.Importantly, tolerance errors at each fiducial alignment step areadditive. That is, first the selective emitter pattern is definedrelative to the fiducials within a certain tolerance followed by themetal deposition also positioned relative to the fiducials with adifferent tolerance. To ensure alignment of the metal contacts to theselective emitter these tolerances are added and either the selectiveemitter pattern is designed larger than the metal pattern to ensure nometal touches the lower doped regions or a degree of misalignment istolerated. In each case a sacrifice in device efficiency potential isaccepted due to higher contact resistance when metal is contacting alower doped region or due to lower current when the selective emitterpattern is designed with large features to ensure metal only contactshighly doped regions. Tighter alignment tolerances enable higherefficiencies.

In general, the aligned placement of two patterns by means of edgealignment requires that the substrate orientation be kept constant (tominimize errors caused by variations in substrate sizes) betweensubsequent deposition steps, which may restrict and complicate substratehandling. Furthermore, if different deposition tools are used forsubsequent layers, the vision systems generally need to base allcalculations on exactly the same locations on a given substrate, whichmay be unrealistic in high throughput manufacturing (generally over 1500substrates/hour). Finally, alignment accuracy is low due to substrategeometries being non-ideal and edges poorly defined.

In view of the foregoing, there is a desire to provide methods ofaligning a set of patterns on a silicon substrate.

SUMMARY

The invention relates, in one embodiment, to a method of aligning a setof patterns on a substrate, the substrate including a substrate surface.The method includes depositing a set of silicon nanoparticles on thesubstrate surface, the set of silicon nanoparticles including a set ofligand molecules including a set of carbon atoms, wherein a first set ofregions is formed where the nanoparticles are deposited and theremaining portions of the substrate surface define a second set ofregions. The method also includes densifying the set of siliconnanoparticles into a thin film wherein a set of silicon-organic zonesare formed on the substrate surface, wherein the first set of regionshas a first reflectivity value and the second set of regions has asecond reflectivity value. The method further includes illuminating thesubstrate surface with an illumination source, wherein the ratio of thesecond reflectivity value to the first reflectivity value is greaterthan about 1.1.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 shows a simplified diagram of a set of diffuse light reflectancecurves for a textured crystalline silicon substrate with a nanoparticlethin film coated with an anti-reflection/passivation layer, inaccordance with the invention;

FIG. 2A shows a simplified diagram comparing the ratio of reflectance ofhighly doped regions to lightly doped regions on a silicon nitridecoated textured crystalline silicon substrate, in accordance with theinvention;

FIG. 2B shows a simplified diagram comparing the ratio of reflectance ofhighly doped regions to lightly doped regions, in which an organicsilicon nanoparticle thin film has first been deposited on a siliconnitride coated textured crystalline silicon substrate to form the highlydoped regions, and then (partially) etched away, followed by thedeposition of a silicon nitride layer, in accordance with the invention;

FIG. 3 shows a simplified diagram of a front contact solar cell with aselective emitter and an additional layer acting as back surface field,in accordance with the invention;

FIG. 4 shows a simplified diagram of a back-contact solar cell, inaccordance with the invention;

FIG. 5A shows a simplified diagram of a pyramid-textured silicon solarcell substrate surface as analyzed by AES (Auger Electron Spectroscopy),where a silicon nanoparticle fluid was first deposited, densified, andthen (partially) removed, in accordance with the invention;

FIG. 5B shows a simplified composite diagram comparing both the grayvalue contrast ratio and normalized efficiency to a silicon nanoparticlefilm thickness after deposition, densification, and (partial) removal,in accordance with the invention;

FIGS. 6A and 6B show a simplified set of diagrams of an apparatus forsuperimposing a set of metal contacts on a set of highly doped regionson a crystalline solar cell substrate, in accordance with the invention;

FIG. 7 shows a simplified diagram comparing ratio of reflectance at 470nm to gray value contrast, in accordance with the invention;

FIG. 8 shows a simplified diagram comparing angle of illumination at 470nm to gray value contrast, in accordance with the invention; and

FIG. 9 shows a simplified diagram comparing illumination wavelength tocontrast, in accordance with the invention.

DETAILED DESCRIPTION

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps and/orstructures have not been described in detail in order to notunnecessarily obscure the present invention.

In general, an anti-reflective coating, such as silicon nitride, isdeposited on a silicon substrate surface in order to maximize the lightavailable to be converted to electrical energy. Creating interferenceand thus canceling two out-of-phase reflected waves, the thickness ofthe anti-reflective coating is generally optimized for a particularwavelength and a particular surface reflectivity. In addition, thesurface of the silicon substrate may be treated to minimize reflection.Any “roughening” of the surface reduces reflection by increasing thechances of reflected light bouncing back onto the surface, rather thanout to the surrounding air.

However, as previously described, the general lack of visual boundariesbetween highly doped and lightly doped regions in a multi-doped junctionmake alignment of metal contacts problematic.

In an advantageous manner, by measuring a ratio of reflectance within aspecific wavelength region between a first highly doped region formed bythe deposition of an organic silicon ink and a second lightly dopeddiffused region, a subsequent pattern, such as a set of metal contacts,may be deposited in a manner optimized for each individual solar cellsubstrate. More specifically, a gray scale image may be generated withdigital imaging of portions of the solar cell substrate surface withboth highly doped and lightly doped areas. This gray value image maythen be segmented into common regions which have substantially similardopant strength.

For example, in one method of forming a highly doped region, a set ofsilicon nanoparticles is first dispersed in a set of organic solvents,deposited in a pattern, densified to form a thin film, and then(partially) removed prior to the deposition of an anti-reflectioncoating. It has been observed that deposited silicon nanoparticles, whencombined with a standard dopant diffusion process such as exposure toPOCl₃ gas at high temperature, allow a multi-doped junction to beformed. The solar cell substrate is generally heated up to around 800°C. to 900° C. Nitrogen is then flowed through a bubbler to form acarrier gas for the POCl₃. The formed gaseous POCl₃, after being mixedwith O₂, deposits PSG (P₂O₅) on the solar cell substrate surface which,in turn, produces phosphorous atoms that diffuse into the crystal Silattice and SiO₂ that forms on the solar cell substrate surface.Subsequently, SiO₂ may be selectively etched by aqueous solutions ofhydrofluoric acid (HF) and/or ammonium fluoride (NH₄F) containingsolutions.

One method of depositing the layer of nanoparticles onto the siliconsubstrate surface is the deposition through the use of a colloidaldispersion (ink) or paste. Generally, colloidal dispersions of Group IVnanoparticles are possible because the interaction of the particlesurface with the solvent is strong enough to overcome differences indensity, which usually result in a material either sinking or floatingin a liquid. That is, smaller nanoparticles disperse more easily thanlarger nanoparticles. Commonly, particle loadings may be around 3 Wt. %.In contrast, if the particle loading substantially increases above about10 Wt. %, the colloidal dispersion thickens into a highly viscous ink orpaste.

In general, the Group IV nanoparticles are transferred into thecolloidal dispersion under a vacuum, or an inert substantiallyoxygen-free environment. In addition, particle dispersal methods andequipment such as sonication, high shear mixers, and high pressure/highshear homogenizers may be used to facilitate dispersion of thenanoparticles in a selected organic solvent or mixture of solvents. Thatis, mixtures that contain carbon molecules.

Examples of solvents include alcohols, aldehydes, ketones, carboxylicacids, esters, amines, organosiloxanes, halogenated hydrocarbons, andother hydrocarbon solvents. In addition, the solvents may be mixed inorder to optimize physical characteristics such as viscosity, density,polarity, etc.

In addition, in order to better disperse the Group IV nanoparticles inthe colloidal dispersion, nanoparticle capping groups may be formed withthe addition of organic compounds, such as alcohols, aldehydes, ketones,carboxylic acids, esters, and amines, as well as organosiloxanes.Alternatively, capping groups may be added in-situ by the addition ofgases into the plasma chamber. These capping groups may be subsequentlyremoved during the sintering process, or in a lower temperature pre-heatjust before the sintering process.

For example, bulky capping agents suitable for use in the preparation ofcapped Group IV semiconductor nanoparticles include C4-C8 branchedalcohols, cyclic alcohols, aldehydes, and ketones, such astertiary-butanol, isobutanol, cyclohexanol, methyl-cyclohexanol,butanal, isobutanal, cyclohexanone, and oraganosiloxanes, such asmethoxy(tris(trimethylsilyl)silane)(MTTMSS), tris(trimethylsilyl)silane(TTMSS), decamethyltetrasiloxane (DMTS), and trimethylmethoxysilane(TMOS).

Once formulated, the colloidal dispersion may be applied to a substrateand subjected to a heat treatment in order to sinter the Group IVnanoparticles into a densified conductive film and consequently enablethe diffusion of a dopant into the substrate. Examples of applicationmethods include, but are not limited to, roll coating, slot die coating,gravure printing, flexographic drum printing, screen printing, andinkjet printing methods, etc.

Additionally, various configurations of doped Group IV nanoparticlecolloidal dispersions can be formulated by the selective blending ofdoped, undoped, and/or differently doped Group IV nanoparticles. Forexample, various formulations of blended Group IV nanoparticle colloidaldispersions can be prepared in which the dopant level for a specificlayer of a junction is formulated by blending doped and undoped Group IVnanoparticles to achieve the requirements for that layer. Alternatively,the blended Group IV nanoparticle colloidal dispersions may be used tocompensate for substrate defects, such as the passivation of oxygenatoms in order to reduce undesirable energy states.

However, solutions of hydrofluoric acid (HF) and/or ammonium fluoride(NH₄F) used to remove SiO₂, as previously described, react with Si—O(silicon-oxygen) bonds substantially faster than Si—Si (silicon-silicon)or Si—C (silicon-carbon) bonds, thus oxides are stripped, while siliconand silicon carbide remain. Consequently, a set of thin non-epitaxialcontinuous or discontinuous zones comprising sets of silicon-organicatomic bonding (silicon and/or silicon carbide) may be formed on thesolar cell substrate surface.

While not wishing to be bound by theory, the inventors believe thatthese thin silicon-organic zones alter the reflectivity of a highlydoped region by at least one of two mechanisms. First, in comparison toregions without silicon-organic residue, the addition of a thin organiclayer effectively alters the thickness of the anti-reflective coating atthat point, which in turn alters the reflected wave interference patternfor aggregate highly doped region surface. Second, the thinsilicon-organic residue regions form small superstructure patterns abovethe substrate surface. An anti-reflective coating, which tends to bedeposited conformally on the substrate surface, will also tend to matchthe superstructure pattern. The superstructure pattern alters theaggregate reflective index of the aggregate highly doped region surfaceby altering the average steepness of the pyramid angles of the texturedsurface.

Referring to FIG. 1, a simplified diagram showing a set of diffuse lightreflectance curves for a textured crystalline silicon substrate with ananoparticle thin film coated with an anti-reflection/passivation layer,as may be used in a solar cell, in accordance with the invention.Incident wavelength 102 is shown on the horizontal axis, while measuredreflectance percentage 104 is shown on the vertical axis.

Here, crystalline silicon substrates are first pre-cleaned in a sulfuricacid solution. In addition, in order to reduce reflection, a pyramidtexture is then added by treating the substrates in a solution ofdeionized H₂O, IPA (isopropanol), and KOH (potassium hydroxide). Next, asilicon nanoparticle thin film pattern is then deposited on the siliconsubstrate that subsequently defines the highly doped regions. Afterdrying at elevated temperatures in an inert atmosphere, the crystallinesilicon substrates are diffused in an atmosphere of POCl₃, N₂, and O₂,as previously described. The residual PSG (phosphosilicate) glass layeris subsequently removed by a BOE (buffered oxide etch) cleaning step.Any portion of the silicon nanoparticle film that is converted to achemical composition substantially similar to PSG will also be removed.However, silicon-organic regions remain, as previously described.

Generally if the chemical etch period is insufficiently long (less thanabout 30 seconds) a PSG oxide thickness of between about 300 nm and 1200nm remains on the solar cell substrate surface. Consequently, althoughthe contrast between highly doped and lightly doped regions may besufficiently good to ascertain a pattern, the corresponding solar cellefficiency is reduced due to a relatively high reflectivity as well ashigh series resistance caused by insufficient removal of the PSG layer,which is a non-conductive oxide. In contrast, if the chemical etchperiod is too long (greater than about 1 hour), the contrast betweenhighly doped and lightly doped regions is poor, increasing thelikelihood of metal contact misalignment. However, in an advantageousmanner, if the chemical etch period is from about 1 minute to about 10minutes, silicon-organic containing regions of thickness between 5 nmand 400 nm remain on the silicon substrate surface with sufficientreflective contrast to lightly doped regions.

In a next step, in order to minimize reflection and to optimize surfacepassivation, an anti-reflection coating and passivation layer of siliconnitride (Si₃N₄ and other non-stochiometric ratios of Si and N) isdeposited on the silicon substrate in an ambient of silane, ammonia,nitrogen, and optionally hydrogen. Here, the reflective index of theSi₃N₄ layer is between about 1.90 and about 2.10, with a thickness ofbetween 40 nm and about 120 nm.

Next, the location of the highly doped regions is determined based ontheir contrast with respect to lightly doped regions, and the metalcontacts are deposited using a deposition device, such as a screenprinter. Here, light is projected onto the textured substrate surfaceand the diffuse reflection is subsequently analyzed. Diffuse reflectionis generally the reflection of light from an uneven surface such as atextured silicon substrate. Consequently, the reflected light spreadsover the hemisphere surrounding the surface (2π steradians).

Curve 110 shows the reflectance of a textured silicon substrate with anoxidized silicon nanoparticle thin film of 1200 nm thickness, as can beused to form a heavily doped region as previously described, coated witha silicon nitride layer of 60 nm thickness. The reflectance at 470 mn is18.2% and at 540 nm is 15.9%.

Curve 108 shows the reflectance of a textured silicon substrate with anoxidized silicon nanoparticle thin film of 500 nm thickness, as can beused to form a heavily doped region as previously described, coated witha silicon nitride layer of 115 nm thickness. The reflectance at 470 mnis 9.3% and at540 nm is 9.7%.

Curve 107 shows the reflectance of a textured silicon substrate with asilicon nanoparticle thin film of less than about 30 nm thickness, ascan be used to form a heavily doped region as previously described,coated with a silicon nitride layer of 115 nm thickness. The reflectanceat 470 mn is 3.4% and at 540 nm is 0.85%.

Curve 105 shows the reflectance of a textured silicon substrate withouta silicon nanoparticle thin film, as may be used to form a lightly dopedregion as previously described, coated with a silicon nitride layer of115 nm thickness. The reflectance at 470 mn is ˜2.7% and at 540 nm is˜0.61%.

As can be seen, for a first wavelength range 112 between 375 nm and 600nm, and a second wavelength range 114 between 700 nm and 800 nm, thereare sharp differences in reflectance. For example, the reflectance ofcurve 107 is similar in shape to the reflectance of curve 105, whichshows a silicon substrate coated with silicon nitride layer only, but isshifted slightly toward higher wavelengths. While not wishing to bebound by theory, the inventors believe that a similar result would beexpected if the silicon nitride layer was slightly thicker or if theangle of the pyramid texturing was steeper.

Referring to FIG. 2A, a simplified diagram comparing the ratio ofreflectance of highly doped regions to lightly doped regions on atextured silicon nitride coated crystalline silicon substrate, inaccordance with the invention. Incident wavelength 202 is shown on thehorizontal axis, while ratio of reflectance curves of highly dopedregions to lightly doped regions 204 is shown on the vertical axis.

Curve 212 shows the reflectance ratio of a region with an oxidizedsilicon nanoparticle thin film of 1200 nm thickness to a region withouta silicon nanoparticle thin film on a textured silicon substrate coatedwith a silicon nitride layer of 60 nm thickness. The ratio ofreflectance at 470 nm is about 9.8 and at 540 nm is about 34.2.

Curve 210 shows the reflectance ratio of a region with an oxidizedsilicon nanoparticle thin film of 500 nm thickness and a region withouta silicon nanoparticle thin film on a textured silicon substrate coatedwith a silicon nitride layer of 115 nm thickness. The ratio ofreflectance at 470 nm is 4.0 and at 540 nm is 20.3.

Curve 208 shows the reflectance ratio of a region with an oxidizedsilicon nanoparticle thin film of less than about 30 nm thickness and aregion without a silicon nanoparticle thin film on a textured siliconsubstrate coated with a silicon nitride layer of 115 nm thickness. Theratio of reflectance at 470 nm is 1.3 and at 540 nm is 1.4.

As shown in FIG. 1, for a first wavelength range 112 between 375 nm and600 nm, and a second wavelength range 114 between 700 nm and 800 nm, thereflectance ratio of highly doped regions to lightly doped regions issignificantly larger than for remaining wavelengths, resulting in highercontrast for wavelength range 112. Wavelengths resulting in a contrastof below 1.3 should be avoided, including wavelengths between 620 nm and680 nm, which are most commonly used in the production of solar cells.

Referring to FIG. 2B, a simplified diagram comparing the ratio ofreflectance of highly doped regions to lightly doped regions, in whichan organic silicon nanoparticle thin film has first been deposited toform the highly doped regions, and then (partially) etched away for 2minutes as previously described, followed by the deposition of a siliconnitride layer, in accordance with the invention. Incident wavelength 202is shown on the horizontal axis, while reflectance ratio 214 is shown onthe vertical axis. As can be seen, for a first wavelength range 112between 375 nm and 600 nm, and a second wavelength range 114 between 700nm and 800 nm, there are sharp differences in reflectivity.

Referring now to FIG. 3, a simplified diagram of a front contact solarcell with a selective emitter and an additional layer acting as backsurface field (BSF). As previously described, in common solar cells, thevisual boundaries between highly doped and lightly doped emitter regionsmay be difficult to determine, making aligned deposition of a set ofmetal contacts difficult.

Emitter 306 may be p-type (e.g., boron doped) or n-type (e.g.,phosphorous doped) and could be formed by various methods, which includebut are not limited to gas phase diffusion (such as e.g. using POCl₃ gasas phosphorous source or BBr₃ as boron source), solid source diffusion,or inline processes which typically use liquid dopant sources such ase.g. phosphoric acid.

Above the emitter 306 (which is also typically coated with ananti-reflection coating 304) is a front metal contact, comprising a setof fingers 305 (here with a width of about 130 um silver) and a set ofbusbars 303 (here with a width of about 1500 um silver). Typically madeout of silver paste with added glass frit, the front metal contact isoptimized to extract the charge carriers (here electrons) created in thesilicon substrate 308 when light is absorbed. The front metal contact isalso typically configured with a reduced horizontal surface area (thusminimizing losses due to shading, which reduces the generated current),and an increased cross-sectional volume (thus reducing the seriesresistance of the device, which tends to increase the efficiency of thedevice).

In general, untreated silicon substrates often reflect more than 30% ofincident light. Consequently, in order to reduce this reflected energyand thus directly improve efficiency, the silicon substrate is generallytextured and optimized with anti-reflective coatings 304 (e.g., siliconnitride (Si₃N₄), etc.). In addition, anti-reflective coating 304 alsohelps passivate the surface of emitter 306, both reducing the impact ofcontamination of the substrate bulk from external sources, as well assubstantially reducing minority carrier recombination caused by danglingSi bonds or imperfections in the doped substrate 308 surface.

In addition, on the back-side of silicon substrate 308 often is aheavily doped region (of the same type as the substrate) which creates aBSF (back surface field) 310. Minimizing the impact of rear surfacerecombination, a properly configured BSF tends to repel those minoritycarriers that are generated closer to the back-side, resulting in higherlevels of minority carrier concentrations in the substrate absorber andhigher device efficiencies. For example, Al (aluminum) or B (boron) maybe added to a p-type substrate to form a BSF layer. In contrast, for ann-type substrate, P (phosphorous) may be added to form a BSF layer. Inaddition, silver (Ag) or silver/aluminum pads are generally inserted inthe back-side in order to facilitate soldering for interconnection intomodules.

Referring now to FIG. 4, a simplified diagram of a back-contact solarcell, in accordance with the invention. In a common configuration, a setof p-type (emitter) regions 412 and a set of n-type base contact regions416 are diffused into an n-type (phosphorous doped) silicon substrate408. Optionally, a surface passivation layer 410 of silicon nitride orsilicon oxide can be deposited on the back side of the surface. In orderto extract the charge carriers, an emitter metal contact 402 isdeposited over the set of p-type regions 412, and a base metal contact411 is deposited over the set of n-type regions 416.

In addition, a front-side layer 404 comprised of an FSF (front-surfacefield) and an anti-reflective coating (as previously described) is alsodeposited. The FSF is similar in function to a BSF in that it tends torepel minority carriers (here electrons) from the front of the solarcell thus improving passivation quality of the front surface.

EXPERIMENT 1

A crystalline silicon substrate was first pre-cleaned in a sulfuric acidsolution and then textured by treating the substrates in a solution ofH₂O, IPA, and KOH. After cleaning and rinsing steps in SC-2 (a mixtureof H₂O, HCl (hydrochloric acid) and H₂O₂ (hydrogen peroxide)), piranha(a mixture of sulfuric acid (H₂SO₄) and H₂O₂), BOE, and H₂O,respectively, a silicon nanoparticle fluid comprising 4 Wt. % siliconnanoparticle in a set of organic solvents, was deposited on thecrystalline silicon substrate. After baking at a temperature of 600° C.in an inert atmosphere for a time period of 3 minutes in order todensify the film and evaporate solvent molecules, the crystallinesilicon substrate was exposed to a dopant source in a diffusion furnacewith an atmosphere of POCl₃, N₂, and O₂, at a temperature of about 925°C. and for a time period of about 100 minutes. The residual PSG glasslayer was subsequently removed by a BOE cleaning step for 5 minutes.During this step, the thickness of the silicon nanoparticle thin filmwas reduced from about 1200 nm to about 30 nm, and a layer ofsilicon-organic residue remained behind.

Next, the crystalline silicon substrate was analyzed using AugerElectron Spectroscopy (AES). In general, AES is a technique fordetermining the composition of the top 5-10 nm of a surface. Incidentelectrons of energy 3-20 keV tend to cause high binding energy coreelectrons from atoms in the top surface of a sample (here a siliconsubstrate) to be ejected. The atom then relaxes via a lower bindingenergy valence electrons dropping into the hole at the core state. Thisquantized relaxation energy is transferred to kinetic energy of anejected electron which can be detected. The kinetic energy of theejected electron is characteristic of the element from which it wasemitted, and can thus be used to identify the element.

Here, although the PSG glass layer and the deposited siliconnanoparticle thin film were substantially removed, a large fraction ofcarbon atoms were detected (along with residual silicon nanoparticles),sufficient to alter the reflectivity of the highly doped region withrespect to the lightly doped regions of the crystalline siliconsubstrate surface.

Referring now to FIG. 5A, a simplified diagram of a pyramid-texturedsilicon solar cell substrate surface as analyzed by AES, where a siliconnanoparticle fluid was first deposited, densified, and then (partially)removed as previously described, in accordance with the invention. Ingeneral, a textured silicon solar cell substrate surface 502 has pyramidformations of varying width and height. In regions of the pyramid tips508, about 18 atom % carbon was measured. At the pyramid midpoints 506,about 15 atom % carbon was measured. And at the pyramid valleys 504,about 34 atom % carbon was measured. After removal of ˜5 nm of surfacematerial by ion milling using Ar, there was still approximately 67 % ofthe original carbon atomic concentration remaining, thus demonstratingthat the carbon signal was not due to advantageous surface carbon, whichcould contribute to carbon signal prior to ion milling in AEStechniques.

EXPERIMENT 2

A set of crystalline silicon substrates was first pre-cleaned in asulfuric acid solution and then textured by treating the substrates in asolution of H₂O, IPA, and KOH. After cleaning and rinsing steps in SC-2(a mixture of H₂O, HCl (hydrochloric acid) and H₂O₂ (hydrogenperoxide)), piranha (a mixture of sulfuric acid (H₂SO₄) and H₂O₂), BOE,and H₂O, respectively, a silicon nanoparticle fluid comprising 4 Wt. %silicon nanoparticle in a set of organic solvents, was deposited on eachtextured crystalline silicon substrate. After baking at a temperature of600° C. in an inert atmosphere for a time period of 3 minutes in orderto densify the film, the crystalline silicon substrates were exposed toa dopant source in a diffusion furnace with an atmosphere of POCl₃, N₂,and O₂, at a temperature of about 925° C. and for a time period of about100 minutes. Next, the substrates were subjected to a BOE cleaning stepof varying time period. The degree of removal of the residual PSG glasslayer and the thickness of the densified silicon nanoparticle film werecontrolled by varying the period of exposure to BOE from 0 minutes to 60minutes. Greater etchant exposure corresponds to a thinner densifiedsilicon nanoparticle film and a more thorough removal of the PSG glasslayer. In addition, an etchant exposure greater than 10 minutessubstantially removes the thinner densified silicon nanoparticle film.

Referring to FIG. 5B, a simplified composite diagram comparing both thegray value contrast ratio obtained for illumination at 470 nm andnormalized efficiency to a silicon nanoparticle film thickness afterdeposition, densification, and (partial) removal, in accordance with theinvention. Normalized efficiency refers to a solar cell efficiency valuethat is normalized to the efficiency of a device with a thickness of adensified silicon nanoparticle film that has not been exposed to anetchant.

In general, gray value contrast ratio is quantified by the ratio of grayscale value of a diffuse light sensor pixel of interest to that of abackground pixel. The gray value is generally directly related to thenumber of photons with appropriate wavelength arriving on a sensor ingiven unit time. Areas on the solar cell substrate surface with higherreflectivity for a given configuration relative to the diffuse lightsensor (i.e. incidence angle, illumination, light wavelength) will behigher on a gray scale. Demarcation between areas with high and lowreflectivity (contrast) are best defined when the difference isgreatest.

Thickness 510 in nm is shown on the horizontal axis, gray value contrastratio 514 of gray values of highly doped regions to lightly dopedregions obtained for illumination at 470 nm is shown on the leftvertical axis, while normalized cell efficiency (power out vs. power in)512 is shown on the right vertical axis. Curve 516 compares gray valuecontrast ratio to thickness, while curve 518 compares normalizedefficiency to thickness of the densified silicon nanoparticle film. Ascan be seen, as the thickness decreases from about 1200 nm to about 10nm, normalized efficiency increases (because of the ability to form abetter ohmic contact with a metal contact), while the gray valuecontrast ratio of highly doped regions to lightly doped regionsdecreases (corresponding to a thinner silicon organic film residue, aspreviously discussed).

Referring now to FIG. 6, a simplified diagram of an apparatus forsuperimposing a set of metal contacts on a set of highly doped regionson a crystalline solar cell substrate in accordance with the invention.

Initially, a solar cell substrate 604 with a set of highly dopedregions, such as for a selective emitter or a back-contact solar cell,is positioned on a substrate intake transport apparatus 603 (e.g.,conveyor belt, etc.). Solar cell substrate 604 is then positioned inpattern detection apparatus 606 in order to determine the axial (x andy) and angular position (theta) of known landmarks within the pattern ofthe highly doped regions. For example, if the set of highly dopedregions is patterned as a set of busbars and fingers, the set of knownlandmarks may be specific intersections of a busbar to a finger at welldefined locations. Alternatively, unique landmarks can be added to thepattern of highly doped regions. These landmarks may be used as uniquefeatures within the patterns of highly doped regions for the imagerecognition step.

Once determined, the locations of the landmarks within the set of highlydoped regions are transmitted to metal deposition apparatus 610 (such asa screen printer which may include squeegee 612). After solar cellsubstrate 604 is positioned in metal deposition apparatus 610 (here viaturntable 608), the printing screen of metal deposition apparatus 610 isadjusted axially and angularly in order to align the set of metalcontacts to be deposited onto the set of highly doped regions. Afterdeposition of the metal contacts onto the set of highly doped regionsusing squeegee 612, the solar cell substrate with the set of metalcontacts [not shown] is then positioned on substrate outtake transportapparatus 611, where it may be transported for additional processing.

In one configuration, substrate 604 is placed in the pattern detectionapparatus 606 of an Applied Materials Baccini screen printer tool with avision system based on three high-resolution cameras and imagerecognition software. Initially, the image recognition software isinstructed to look for a specific set of patterns or landmarks. Here,the image recognition software vision libraries are based on CognexVision Pro and may include several algorithms such as ‘PatternMatching’, PatMax', and PatCAD'. The first two algorithms record animage of a model pattern of best possible quality and process it intocharacteristic geometric features (i.e. lines, circles) that are usedfor image recognition, whereas the last algorithm allows loading adesign generated with CAD software, which represents the model patternin an ideal case.

A CAD pattern algorithm in particular may be advantageous because itallows the pattern recognition software to search for a well-defined CADpattern rather than a manually defined search area, which enhances theplacement accuracy and resilience to contrast and pattern variations. Inaddition, for the calculation of the required screen rotation, thecenter coordinates of the respective search areas with respect to thecenter of the entire pattern generally need to be accurately known. Inan advantageous manner, these coordinates tend to be well defined forthe CAD pattern, whereas with the remaining algorithms and manualselection of a search area the center coordinates have to be estimatedcausing an additional placement error.

In general, in order to be identified, the set of patterns should bechosen such that it is located within the field of view of each cameraused for the vision system. Pattern recognition generally requires theavailability of unique features within the pattern, which definedistinct locations in x and y direction. These features can either becontained in the first pattern itself, or may be added as uniquealignment features without impacting the device performance. Next, in anadvantageous manner, the solar cell substrate is illuminated at awavelength preferably between about 375 nm and about 600 nm and between700 nm and 800 nm, and most preferably at 470 nm or 540 nm.

The inventors believe that these wavelength ranges are optimized formaximum contrast of the highly doped regions with respect to the lightlydoped regions of the crystalline solar cell substrate (background).Achievable gray value contrast and reflectance ratios are higher for 540nm (see FIG. 7), but in common applications, substrate texturing andanti-reflection coatings are optimized such that absolute reflectance isminimal close to this wavelength. Therefore, the while 540 nm hassuperior contrast ratio compared to 470 nm the, 470 nm has superiorsignal-to-noise ratio. The optimum configuration balances these factorsin an advantageous manner depending on the absolute reflectance of thesubstrate and illumination level. Furthermore, the efficiency of greenLEDs is commonly lower than of blue LEDs, making 470 nm a favorableillumination wavelength. In general, wavelengths between 700 and 800 nmyield considerable contrast as well, but even though the common CCD(charge-coupled device) cameras have considerable sensitivity in thiswavelength range, the peak response is usually between 500 and 550 nmmaking this wavelength region less advantageous.

In addition, the angle of the illumination source may be adjusted suchthat it is optimized for maximum contrast when observed with a diffuselight sensor. In this configuration, vertical orientation of theillumination source relative to the lens axis of the diffuse lightsensor yields best results, but in other configurations (depending onsubstrate surface, diffusion properties, etc), the angle can be variedwithin 90 degrees with respect to the substrate surface.

The angle of the diffused light sensor may be adjusted such that it isoptimized to obtain maximum contrast of the highly doped region withrespect to the lightly doped regions on the solar cell substrate. Inthis configuration, a substantially vertical orientation of the lightsensor with respect to substrate surface yields best results, but inother configurations (depending on substrate surface, diffusionproperties, etc), angle can be varied to 90 degrees with respect to thesubstrate surface.

In addition, the intensity of the illumination source may be adjusted toobtain sufficient contrast. In this configuration, 4 linear bars of blueLEDs of 470 nm have been used, with 4×96 LEDs in each bar and anadjustable total light intensity of 15.5 W maximum. Similar linear barsof white LEDs covered with a green color filter have been used toproduce illumination at 540 nm. In an alternate configuration,ring-shaped illumination sources around each camera lens and coaxialillumination may be used as functional alternatives, since both maintainthe incident and reflected light paths nearly parallel to the substrateplane normal.

In general, the diffuse light sensors are selected such that they aresensitive to the wavelengths of the appropriate illumination source. Inthis configuration, video cameras are used with appropriate CCD chips.The resolution needs to be sufficient to resolve all relevant featuresof the pattern used for the alignment. The field of view needs to belarge enough to image all relevant features of the pattern used for thealignment. In this configuration, SONY XCD-SX90 CCD cameras were used,together with Tamron lenses CCTV 21HA (with a focal length of 50 mm).The resolution was 1280×960 pixels. The field of view at approximately155 mm operating distance was about 20 mm×15 mm. Band pass filtersoptimized to the wavelength range of the illumination source may be usedto filter out background light and obtain better contrast.

Typically, the requirements for the placement accuracy of the substratecontaining the pattern are that the areas of interest are within thefield of view of the cameras. In our case this corresponds to an area of20 mm×15 mm. The sizes of the features that are used for imagerecognition are 5 mm×5 mm. Therefore, the substrate can be placed within±7.5 mm in both directions.

In general, vision system software has parameters needed in order todetect the model pattern of interest in the recorded image. For example,exposure, brightness, contrast may need to be adjusted in order toobtain maximum contrast. In general, the values depend on the givenillumination source and position, camera type and position, substratesurface, and pattern composition, etc. The values should further beadjusted such that the gray scale contrast ratio is large enough toallow for pattern recognition. In this configuration, successful patternrecognition requires this ratio to be larger than 1.3. Values as low as1.1 may be sufficient to allow for successful pattern recognition.

In general, vision software searches for a match of the model pattern tothe detected image. The geometric constraints of the model can beadjusted to allow for asymmetric or symmetric scaling. In this example,the scaling was allowed to be between 0.8 and 1.6, and symmetric andasymmetric scaling was used, depending on the pattern fidelity of theprinted pattern.

In addition, gain should be adjusted according to the image quality ofthe recorded image. Grain generally defines the number of pixels overwhich the transition between two regions is assumed to occur. Inaddition, low contrast substrates require high exposure and contrastsettings in the software, leading to grainier images. Grain adjustmentscan enable pattern recognition for such cases. The settings in our caserange from 1 to 12.

Angle adjustments may also need to be made in order to exclude patternrecognition for rotations in excess of a certain angle. In this case,patterns within ±10 degrees were searched.

In the next step, the screen translation/rotation and alignment withrespect to the previously deposited pattern is calculated. In general,for pattern alignment, although at least two coordinates need to bedefined for a calculation of screen translation and rotation, theaddition of a third coordinate may improve the accuracy of the alignmentprocess. In this configuration, the alignment was based on threecoordinates. The coordinates used for the pattern alignment calculationsare defined as the geometric center of the model pattern, eitherdetermined by a model image, or by a CAD drawing.

The substrate may then be inspected with an optical vision tool (such asan optical microscope or Micro-Vu Vertex 320 tool as described below),and the respective coordinates of the landmark features as discussedabove may be determined, and then transmitted to or entered into thesoftware as nominal positions. In general, the vision software comparesthese nominal locations with the ones determined in the imagerecognition process and calculates a translation and rotation for thescreen to guarantee alignment of the two patterns.

In this example, a test sample was printed and inspected using the samevision tool. Any remaining axial and angular offsets that depend onmechanical parameters (screen location, squeegee location, etc.) may becorrected in the vision software. Once determined, the location of theknown landmarks within the set of highly doped regions was transmittedto metal deposition apparatus 610, which included squeegee 612 aspreviously described. After adjusting the setup and parameters for theimage recognition and pattern alignment process, a layer of silvercontaining glass frit to fire through the Si₃N₄ layer was printed on topof the previously deposited layer defining the highly doped region.Consequently, successive samples may be printed within the achievableaccuracy of the vision system. Here, the sample-to-sample accuracy hasbeen shown to be better than ±16 μm.

After a first drying step to remove excessive solvents, the backside ofthe cells was printed with an Ag/Al contact layer, and after anotherdrying step, a BSF layer comprised of aluminum was printed.Subsequently, the solar cell substrate 604 was processed in a thirddrying step, followed by the contact formation which may be done byco-firing the cells in a belt furnace.

Next, the crystalline solar cell substrate was processed by a solar cellsubstrate edge isolation apparatus, usually including a laser whereby agroove is continuously scribed completely through the emitter layer.

Optionally, in order to check the quality of the actual metaldeposition, the alignment of the deposited metal contact with respect tothe highly doped region pattern may be determined. In general, thedeposition mechanism of metal deposition apparatus 610 (here squeegee612) must be physically aligned and positioned to an initial knownposition and angle. The metal deposition apparatus 610 in turn offsetsfrom this position and angle with pattern information from the visionsoftware in order to actually deposit the metal on the highly dopedregion. However, the actual position and angle tend to change due tomanufacturing conditions such as wear and temperature fluctuations,which in turn causes the position of the resulting deposited pattern todrift. In general, it is desirable to analyze pattern alignment duringmanufacturing at a rate at least twice that of the periodicity ofmachine offset variation within ±5 μm.

In an advantageous manner, using a vision apparatus, such as a Micro-VuVertex, the actual drift between the deposited metal and the highlydoped region pattern may be automatically determined by varying theillumination angle and intensity of a light source. For example, theMicro-Vu Vertex projects an illumination source optimized to yieldmaximum contrast. In this configuration, the standard red light sourceof the vision system was replaced by white LEDs, which tend to have astrong contribution in the required wavelengths range below 600 nm,specifically 470 nm and 540 nm.

Here, the substrate surface is first illuminated with a whiteillumination source with an incident power of about 50 mW/cm² to about400 mW/cm² and an incident angle of 2-20 degrees from the substratesurface normal vector in order to detect known landmarks of the highlydoped regions. It has also been determined experimentally that for aautomated vision tool with variable angle ring illumination, detectionof the highly doped regions can be improved with the simultaneousillumination of an additional light source at an incident angle of60-80° from the substrate surface normal vector. Next, the substratesurface is illuminated with a light source with an incident power ofabout 1 mW/cm² to about 20 mW/cm² and an incident angle of 0-85 degreesfrom the substrate surface normal vector in order to detect knownlandmarks of the deposited metal. After recognition of the knownlandmarks from both the selective emitter pattern and metal contact, theactual axial and angular position between the selective emitter and themetal contact patterns may be determined.

Referring now to FIG. 6B, a simplified diagram of one configuration ofthe pattern detection apparatus of FIG. 6A, in accordance with theinvention. Here, a set of LED (light emitting diodes) banks 624 waspositioned around a set of diffuse light detectors (e.g., cameras, etc.)630A-C. As solar cell substrate 604 was positioned under the set of LEDbanks 624, and light with a wavelength between about 375 nm and about600 nm was projected onto the substrate surface. As previouslydescribed, the uneven surface solar cell substrate 604 causes thereflection 622 to be diffuse. Each detector of the set of diffuse lightdetectors 630A-C was positioned with respect to a corresponding knownlandmark 620A-C as previously described. In this configuration, eachbank of the set of LED banks 624 had 96 blue LEDs with an illuminationwavelength of 470 nm and a maximum intensity of 15.5 W.

In addition, the system was comprised of diffuse light detectors F30A-CSONY XCD-SX90 CCD cameras, with Tamron lenses CCTV 21HA (focal length is50 mm). Each camera was configured with a resolution of about 1280×960pixels and a field of view of about 20 mm×15 mm and operating distanceof about 155 mm. Furthermore, optical filters may be used to optimizethe contrast of the set of highly doped regions to lightly dopedregions. In some cases, it is advantageous to use long pass, short pass,or band pass filters. In general, these filters allow wavelengths of theillumination source to pass, while wavelengths attributed to backgroundlight to be filtered. Here, 470 nm band pass filters were used for 470nm illumination.

Referring to FIG. 7, a simplified diagram comparing ratio of reflectanceto gray value contrast for illumination at 470 nm and 540 nm,respectively, in accordance with the invention. Ratio of reflectance 702is shown on the horizontal axis, gray value contrast ratio at 470 nm 704is shown on the left vertical axis, and gray value contrast ratio at 540nm 706 is shown on the right vertical axis. As can be seen, gray valuecontrast between two proximate pixels on a measured diffused light imageis directly correlated to the observed ratio of reflectance betweenthose two pixels.

Referring to FIG. 8, a simplified diagram comparing angle ofillumination to gray value contrast, in accordance with the invention.In general, the angle of illumination is relative to the lens axis ofthe diffuse light sensor. Angle of illumination in degrees 802 is shownon the horizontal axis, while gray value contrast ratio 804 is shown onthe vertical axis. In general, a substantially high gray scale contrastratio (1.42) is observed when the angle of illumination is about 0.0degrees.

While not wishing to be bound by theory, the inventors believe thatlight is principally scattered in a normal direction in those highlydoped regions with the silicon-organic zones (especially in valleys ofthe pyramid textured surface), whereas in lightly doped regions there isno dominant light scattering direction. Consequently, a maximum contrastmay be measured when the angle of illumination is about 0.0.

Referring now to FIG. 9, a simplified diagram comparing illuminationwavelength to contrast, in accordance with the invention. Light sourcewavelength 902 is shown on the horizontal axis, while gray valuecontrast ratio 904 is shown on the vertical axis. As can be seen, awavelength of both 470 nm and 540 nm produce sufficient contrast betweenthe highly doped and lightly doped regions on the silicon substratesurface, whereas a wavelength of about 655 nm does not show sufficientcontrast. The latter wavelength is in a spectral region which is mostcommonly used in the production of solar cells.

The inventions illustratively described herein may suitably be practicedin the absence of any element or elements, limitation or limitations,not specifically disclosed herein. Thus, for example, the terms“comprising,” “including,” “containing,” etc. shall be read expansivelyand without limitation. Additionally, the terms and expressions employedherein have been used as terms of description and not of limitation, andthere is no intention in the use of such terms and expressions ofexcluding any equivalents of the features shown and described orportions thereof, but it is recognized that various modifications arepossible within the scope of the invention claimed.

Thus, it should be understood that although the present invention hasbeen specifically disclosed by preferred embodiments and optionalfeatures, modification, improvement and variation of the inventionsherein disclosed may be resorted to by those skilled in the art, andthat such modifications, improvements and variations are considered tobe within the scope of this invention. The materials, methods, andexamples provided here are representative of preferred embodiments, areexemplary, and are not intended as limitations on the scope of theinvention. For example, the silicon substrate may include crystallinesubstrates and multi-crystalline substrates.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeinclude the number recited and refer to ranges which can be subsequentlybroken down into subranges as discussed above.

All publications, patent applications, issued patents, and otherdocuments referred to in this specification are herein incorporated byreference as if each individual publication, patent application, issuedpatent, or other document were specifically and individually indicatedto be incorporated by reference in its entirety. Definitions that arecontained in text incorporated by reference are excluded to the extentthat they contradict definitions in this disclosure.

For the purposes of this disclosure and unless otherwise specified, “a”or “an” means “one or more.” All patents, applications, references andpublications cited herein are incorporated by reference in theirentirety to the same extent as if they were individually incorporated byreference. In addition, the word set refers to a collection of one ormore items or objects.

Advantages of the invention include methods of aligning a set ofpatterns on a silicon substrate. Additional advantages include theproduction of solar cells with enhanced efficiency and lower contactresistance.

Having disclosed exemplary embodiments and the best mode, modificationsand variations may be made to the disclosed embodiments while remainingwithin the subject and spirit of the invention as defined by thefollowing claims.

1. A method for manufacturing a photovoltaic cell, comprising:depositing a colloidal dispersion comprising a silicon particle and anorganic solvent on a substrate surface of a substrate, wherein a firstset of regions is formed where the colloidal dispersion is deposited anda second set of regions is formed where the colloidal dispersion is notdeposited; densifying the colloidal dispersion into a thin film, whereinthe first set of regions has a first reflectivity value and the secondset of regions has a second reflectivity value; depositing ananti-reflective coating on the substrate surface; illuminating thesubstrate surface with an illumination source; measuring a reflectedcontrast between the first set of regions and the second set of regions,depositing a fluid on the first set of regions.
 2. The method of claim1, wherein the ratio of the second reflectivity value to the firstreflectivity value is greater than about 1.1.
 3. The method of claim 1,wherein the illumination source has a wavelength between about 375 nmand about 600 nm or between about 700 nm and about 800 nm.
 4. The methodof claim 1, wherein the illumination source has a wavelength of about470 nm or about 540 nm.
 5. The method of claim 1, wherein the colloidaldispersion comprises less than about 10 wt % of the silicon particlebased on the content of the dispersion.
 6. The method of claim 1,wherein the substrate surface is one of a front surface and a backsurface.
 7. The method of claim 1, wherein the fluid is one of a silverpaste and an aluminum paste.
 8. The method of claim 1, furthercomprising illuminating the substrate surface with a first whiteillumination source with a first incident power of about 50 mW/cm² toabout 400 mW/cm² and at a first incident angle of about 2 degrees toabout 20 degrees from a substrate surface normal vector, andilluminating the substrate surface with a second white illuminationsource with a second incident power of about 1 mW/cm² to about 20 mW/cm²and at an second incident angle of about 0 degrees to about 85 degreesfrom the substrate surface normal vector, after illuminating thesubstrate surface with the illumination source.
 9. The method of claim1, further comprising illuminating the substrate surface with a firstwhite illumination source with a first incident power of about 50 mW/cm²to about 400 mW/cm² and at a first incident angle of about 2 degrees toabout 20 degrees from a substrate surface normal vector, and furtherilluminating the substrate surface with a second white illuminationsource with a second incident power of about 50 mW/cm² to about 400mW/cm² and at a second incident angle of about 60 degrees to about 80degrees from the substrate surface normal vector, and illuminating thesubstrate surface with a third white illumination source with a thirdincident power of about 1 mW/cm² to about 20 mW/cm² and at an incidentangle of about 0 degrees to about 85 degrees from the substrate surfacenormal vector, after illuminating the substrate surface with theillumination source.
 10. The method of claim 1, wherein the siliconparticle is a doped or intrinsic silicon nanoparticle.
 11. The method ofclaim 1, wherein the reflected contrast is measured by a diffuse lightsensor with a lens axis, wherein the illumination source issubstantially parallel to the lens axis.
 12. The method of claim 1,wherein the first set of regions has a high dopant concentration and thesecond set of regions has a low dopant concentration.